LOS ANGELES — An AIDS health care group is defending an ad campaign in Los Angeles that links popular dating apps with the spread of sexual transmitted diseases.

The AIDS Healthcare Foundation targets Tinder, a location-based dating app and Grindr, a similar site for gay men, in billboards and bus benches that are aimed at reminding users about the risks of casual sex and offering free STD tests.

“In many ways, location-based mobile dating apps are becoming a digital bathhouse for millennials wherein the next sexual encounter can literally just be a few feet away — as well as the next STD,” Whitney Engeran-Cordova, senior public health director for the foundation, said in a statement.

Billboards show a silhouette of a man labeled “Tinder” face-to-face with a woman’s silhouette labeled “chlamydia.” A silhouette of a man labeled “Grindr” faces a male silhouette labeled “gonorrhea.”

Reaction from the dating apps was swift.

The billboard, put up by The AIDS Healthcare Foundation, also targets Grindr.

Within two hours of the first billboard going up, Grindr pulled commercials that the foundation pays for on the dating site to promote free STD testing, the foundation said.

In less than 24 hours, Tinder sent a cease and desist letter claiming the campaign falsely associates the dating app with the spread of venereal diseases.

“These unprovoked and wholly unsubstantiated accusations are made to irreparably damage Tinder’s reputation in an attempt to encourage others to take an HIV test offered by your organization,” a lawyer for Tinder wrote.

The foundation sent a letter to Tinder denying that it disparaged the company and saying it would not remove the reference to the app.

The campaign is intended to draw attention to rising STD rates that the foundation said coincides with the popularity of the sites that make hook-ups easier, said Michael Weinstein, the foundation’s president.

“It’s logical, if you can be hooked up with someone in an urban area within minutes,” he said, “of course you’re going to have to more STDs.”

Email messages sent to Tinder and Grindr for comment were not immediately returned.

The signs are currently on a dozen billboards in Los Angeles and 45 bus benches, Weinstein said.

The Los Angeles-based foundation, which provides health care to HIV and AIDS patients and free sexual disease testing, plans to also put them up in New York City, Miami and Fort Lauderdale, Florida.